This invention relates in general to overcoated electrophotographic imaging members and more specifically, to overcoating compositions, overcoated electrophotographic imaging members and methods of fabricating and using the overcoated electrophotographic imaging members.
Electrophotographic imaging members, i.e., photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is a good insulator in the dark so that electric charges are retained on its surface. Upon exposure to light, the charge is dissipated in the exposed regions.
An electrostatic latent image is formed on the photoreceptor by first uniformly depositing an electric charge over the surface of the photoconductive layer by one of any suitable means well known in the art. The photoconductive layer functions as a charge storage capacitor with charge on its free surface and an equal charge of opposite polarity (the counter charge) on the conductive substrate. A light image is then projected onto the photoconductive layer. On those portions of the photoconductive layer that are exposed to light, the electric charge is conducted through the layer reducing the surface charge. The portions of the surface of the photoconductive not exposed to light retain their surface charge. The quantity of electric charge at any particular area of the photoconductive surface is inversely related to the illumination incident thereon, thus forming an electrostatic latent image. After development of the latent image with toner particles to form a toner image, the toner image is usually transferred to a receiving member such as paper. Transfer is effected by various means such as, for example, by electrostatic transfer during which an electrostatic charge is applied to the back side of the receiving member while the front side of the member is in contact with the toner image.
The photodischarge of the photoconductive layer requires that the layer photogenerate free charge carriers and transport this charge through the layer thereby neutralizing the charge on the surface. Two types of photoreceptor structures have been employed: One type comprises multilayer structures wherein separate layers perform the functions of charge generation and charge transport, respectively. The other type comprises single layer photoconductors which perform both functions. These layers are formed on an electrically conductive substrate and may include an optional charge blocking and an adhesive layer between the conductive layer and the photoconducting layer or layers. Additionally, the substrate may comprise a non-conducting mechanical support with a conductive surface. Other layers for providing special functions such as incoherent reflection of laser light, dot patterns for pictorial imaging or subbing layers to provide chemical settings and/or a smooth coating surface may optionally be employed.
One common type of photoreceptor is a multilayered device that comprises a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. The charge transport layer can contain an active aromatic diamine molecule, which enables charge transport, dissolved or molecularly dispersed in a film forming binder. This type of charge transport layer is described, for example in U.S. Pat. No. 4,265,990. Other charge transport molecules disclosed in the prior art include a variety of electron donor, aromatic amines, oxadiazoles, oxazoles, hydrazones and stilbenes for hole transport and electron acceptor molecules for electron transport. Another type of charge transport layer has been developed which utilizes a charge transporting polymer wherein the charge transporting moiety is incorporated in the polymer as a group pendant from the backbone of the polymer backbone or as a moiety in the backbone of the polymer. These types of charge transport polymers include materials such as poly(N-vinylcarbazole), polysilylenes, and others including those described, for example, in U.S. Pat. Nos. 4,618,551, 4,806,443, 4,806,444, 4,818,650, 4,935,487, and 4,956,440. The disclosures of these patents are incorporated herein in their entirety.
Charge generator layers comprise amorphous films of selenium and alloys of selenium and arsenic, tellurium, germanium and the like, hydrogenated amorphous silicon and compounds of silicon and germanium, carbon, oxygen, nitrogen and the like fabricated by vacuum evaporation or other suitable techniques for deposition. The charge generator layers may also comprise inorganic pigments of crystalline selenium and its alloys; Group II-VI compounds; and organic pigments such as quinacridones, polycyclic pigments such as dibromo anthanthrone pigments, perylene and perinone diamines, polynuclear aromatic quinones, azo pigments including bis-, tris- and tetrakis-azos; and the like dispersed in a film forming polymeric binder and fabricated by solvent coating techniques.
Phthalocyanines have been employed as photogenerating materials for use in laser printers utilizing infrared exposure systems. Infrared sensitivity is required for photoreceptors exposed to low cost semiconductor laser diode light exposure devices. The absorption spectrum and photosensitivity of the phthalocyanines depend on the central metal atom of the compound. Many metal phthalocyanines have been reported and include, for example, oxyvanadium phthalocyanine, chloroaluminum phthalocyanine, copper phthalocyanine, oxytitanium phthalocyanine, chlorogallium phthalocyanine, magnesium phthalocyanine and metal-free phthalocyanine. The phthalocyanines exist in many crystal forms which have a strong influence on photogeneration.
One design criteria for the selection of the photosensitive pigment for a charge generator layer and the charge transporting molecule for a transport layer is that, when light photons photogenerate holes in the pigment, the holes be efficiently injected into the charge transporting molecule in the transport layer. More specifically, the injection efficiency from the pigment to the transport layer should be high. A second design criterion is that the injected holes be transported across the charge transport layer in a short time; shorter than the time duration between the exposure and development stations in an imaging device. The transit time across the transport layer is determined by the charge carrier mobility in the transport layer. The charge carrier mobility is the velocity per unit field and has dimensions of cm.sup.2 /volt sec. The charge carrier mobility is a function of structure of the charge transporting molecule, the concentration of the charge transporting molecule in the transport layer and the electrically "inactive" binder polymer in which the charge transport molecule is dispersed.
Reprographic machines often utilize multilayered organic photoconductors and can also employ corotrons, scorotrons or bias charging rolls to charge the photoconductors prior to imagewise exposure. Further, corotrons, scorotrons or bias transfer rolls may be utilized to transfer toner images from a photoreceptor to a receiving member. Bias transfer rolls for charging purposes have the advantage that they generally emit less ozone than corotrons and scorotrons. It has been found that as the speed and number of imaging of copiers, duplicators and printers are increased, bias transfer rolls and bias charge rolls can cause serious wear problems to the photoreceptors. Bias transfer rolls and bias charge rolls are known in the art. Bias transfer rolls, which are similar to bias charge rolls, are described, for example in U.S. Pat. No. 5,420,677, U.S. Pat. No. 5,321,476 and U.S. Pat. No. 5,303,014. The entire disclosures of these patents are incorporated herein by reference. As a consequence of the abrasive action of the bias transfer rolls and bias charge rolls, the operating lifetime of conventional photoreceptors is severely reduced. In a test conducted on a normally abrasion resistant non-crosslinked overcoated photoreceptor composition, introduction of bias transfer roll and bias charge roll subsystems cause a greater than eight fold increase in wear of the overcoated photoreceptor. The precise nature of the electrical/abrasive wearing away of the charge transport layer thickness is unknown, but it is theorized that some degradative process involving charge scission of the binder occurs, or in the case of arylamine hole transporting polymers, a reduction in chain lengths causes the polymers to lose their inherent strength.
As described above, one type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a charge blocking layer a charge generating layer, and a charge transport layer. The charge transport layer often comprises an activating small molecule dispersed or dissolved in a polymeric film forming binder. Generally, the polymeric film forming binder in the transport layer is electrically inactive by itself and becomes electrically active when it contains the activating molecule. The expression "electrically active" means that the material is capable of supporting the injection of photogenerated charge carriers from the material in the charge generating layer and is capable of allowing the transport of these charge carriers through the electrically active layer in order to discharge a surface charge on the active layer. The multilayered type of photoreceptor may also comprise additional layers such as an anticurl backing layer, an adhesive layer, and an overcoating layer. Although excellent toner images may be obtained with multilayered belt photoreceptors that are developed with dry developer powder (toner), it has been found that these same photoreceptors become unstable when employed with liquid development systems. These photoreceptors suffer from cracking, crazing, crystallization of active compounds, phase separation of activating compounds and extraction of activating compounds caused by contact with typical organic carrier fluids such as isoparaffinic hydrocarbons, e.g. Isopar, commonly employed in liquid developer inks. These carrier fluids markedly degrade the mechanical integrity and electrical properties of the photoreceptor. More specifically, the organic carrier fluid of a liquid developer tends to leach out activating small molecules, such as the arylamine containing compounds typically used in the charge transport layers. Representative of this class of materials are: N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]4-4,'-diamine; bis-(4-diethylamino-2-methylphenyl)-phenylmethane; 2,5-bis-(4'-dimethylaminophenyl)-1,3,4,-oxadiazole; 1-phenyl-3-(4'-diethylaminostyryl)-5-(4"-diethylaminophenyl)-pyrazoline; 1,1-bis-(4-(di-N,N'-p-methylphenyl)-aminophenyl)-cyclohexane; 4-diethylaminobenzaldehyde-1,1-diphenylhydrazone; 1,1-diphenyl-2(p-N,N-diphenyl amino phenyl)-ethylene; N-ethylcarbazole-3-carboxaldehyde-1-methyl-1phenylhydrazone. The leaching process results in crystallization of the activating small molecules, such as the aforementioned arylamine compounds, onto the photoreceptor surface and subsequent migration of arylamines into the liquid developer ink. In addition, the ink vehicle, typically a C10-C14 branched hydrocarbon, induces the formation of cracks and crazes in the photoreceptor surface. These effects lead to copy defects and shortened photoreceptor life. The degradation of the photoreceptor manifests itself as increased background and other printing defects prior to complete physical photoreceptor failure. The leaching out of the activating small molecule also increases the susceptibility of the transport layer to solvent/stress cracking when the belt is parked over a belt support roller during periods of non-use. Some carrier fluids may also promote phase separation of the activating small molecules, such as arylamine compounds, in the transport layers, particularly when high concentrations of the arylamine compounds are present in the transport layer binder. Phase separation of activating small molecules also adversely alters the electrical and mechanical properties of a photoreceptor. Similarly, single layer photoreceptors having a single active layer comprising photoconductive particles dispersed in a charge transport film forming binder are also vulnerable to the same degradation problems encountered by the previously described multilayered type of photoreceptor when exposed to liquid developers. Although flexing is normally not encountered with rigid, cylindrical, multilayered photoreceptors which utilize charge transport layers containing activating small molecules dispersed or dissolved in a polymeric film forming binder, electrical degradation are similarly encountered during development with liquid developers. Sufficient degradation of these photoreceptors by liquid developers can occur in less than two hours as indicated by leaching of the small molecule and cracking of the matrix polymer film. Continued exposure for several days severely damages the photoreceptor. Thus, in advanced imaging systems utilizing multilayered belt photoreceptors exposed to liquid development systems, cracking and crazing have been encountered in critical charge transport layers during belt cycling. Cracks developing in charge transport layers during cycling can be manifested as print-out defects adversely affecting copy quality. Furthermore, cracks in the photoreceptor pick up toner particles which cannot be removed in the cleaning step and may be transferred to the background in subsequent prints. In addition, crack areas are subject to delamination when contacted with blade cleaning devices thus limiting the options in electrophotographic product design.
Photoreceptors have been developed which comprise charge transfer complexes prepared with polymeric molecules. For example, charge transfer complexes formed with polyvinyl carbazole are disclosed in U.S. Pat. No. 4,047,948, U.S. Pat. No. 4,346,158 and U.S. Pat. No. 4,388,392. Photoreceptors utilizing polyvinyl carbazole layers, as compared with current photoreceptor requirements, exhibit relatively poor xerographic performance in both electrical and mechanical properties. Polymeric arylamine molecules prepared from the condensation or di-secondary amine with a di-iodo aryl compound are disclosed in European patent publication 34,425, published Aug. 26, 1981, issued May 16, 1984. Since these polymers are extremely brittle and form films which are very susceptible to physical damage, their use in a flexible belt configuration is precluded. Thus, in advanced imaging systems utilizing multilayered belt photoreceptors exposed to liquid development, cracking and crazing have been encountered in critical charge transport layers during belt cycling. Still other arylamine charge transporting polymers such as those disclosed in U.S. Pat. No. 4,806,444, U.S. Pat. No. 4,806,443, U.S. Pat. No. 4,935,487, and U.S. Pat. No. 5,030,532 are vulnerable to reduced life because of the highly abrasive conditions presented by imaging systems utilizing bias transfer rolls and/or bias charge rollers.
Protective overcoatings can be somewhat helpful against abrasion. However, most protective overcoatings also fail early when subjected to the highly abrasive conditions presented by imaging systems utilizing bias transfer rolls and/or bias charge rollers. Moreover, many overcoatings tend to accumulate residual charge during cycling. This can cause a condition known as cycle-up in which the residual potential continues to increase with multi-cycle operation. This can give rise to increased densities in the background areas of the final images.
Drum machines employing small diameter drum blanks coated with organic photoreceptors are even more susceptible to degradation since it takes many revolutions of the drum to make a single print. The wear in machines employing bias charge rolls and bias transfer rolls might be as much as 10 micrometers in less than 100,000 revolutions which could translate to as few as 10,000 prints. There is an urgent need for an effective overcoat for these drums. Since the drums are invariably dip coated, one of the requirements for the overcoat material requirements is ease and economical synthesis of the materials and a pot life of several weeks. Pot life is the life of the coating slurry without changes in slurry properties so that the same mixture can be used for several weeks. With coating compositions that ultimately cross link and provide wear protection, there is a danger of initiation of cross linking in the pot itself thereby rendering the remaining material in the pot useless for coating. Since the unused material must be discarded and the pot cleaned or replaced, this waste of material and effort has a significant negative impact on the manufacturing cost. In some instances phase separation of different constituents in the overcoat slurry and or changes in the viscosity impact the pot life of the coating slurry.